The present invention relates to a mold for injection-molding a golf ball and a golf ball manufacturing method, both of which may be advantageously used for molding a golf ball composed of a core encased by a cover of one or more layer, and particularly for forming an outermost cover layer having numerous dimples formed on the surface thereof.
In recent years, there has been a strong desire for golf balls which possess various performance attributes, including not only distance, but also controllability, durability and feel at impact. Satisfying all of these attributes with only one type of material is generally difficult. Hence, the customary practice is to provide the ball with a structure in which a solid core formed of rubber, resin or the like, or a wound core, is encased by a cover of one or more layer, each layer having a particular function. In other words, efforts have been made to achieve a performance which addresses the needs of the player, such as a desire for distance or a desire for controllability, by adjusting the number and thicknesses of the above layers, and also adjusting the formulations, etc. of the materials making up the respective layers.
An injection-molding process is typically used to form the outermost layer of a golf ball having such a structure. Specifically, use is made of a process in which a core or a sphere (referred to below as a “center sphere”) composed of such a core encased by one or more intermediate layer (a layer other than the outermost layer of the cover) is placed in the cavity of a given mold, and a cover-forming material (generally a resin composition; sometimes referred to below simply as a “resin”) is injected between the center sphere and the inner wall of the cavity. In this case, at the same time that the outermost layer is formed, numerous dimples are formed by numerous dimple-forming protrusions which have been provided on the inner wall of the cavity.
Up until now, molds with the structure shown in FIG. 4 have been commonly used when producing golf balls by the above process. FIG. 4, which is a cross-sectional view showing an example of a conventional mold for injection-molding a golf ball, depicts the state prior to injection of the cover-forming material into the mold cavity.
In FIG. 4, a conventional mold 10 is equipped with a mold body 20 having an upper mold half 20a and a lower mold half 20b which part at a golf ball equator-based parting line PL and removably mate to form an interior cavity 3 for molding a golf ball, the cavity 3 having an inner wall with numerous dimple-forming protrusions provided thereon; and support pins 40 which can be extended and retracted within pin insertion holes 50 that communicate with the cavity 3. The support pins 40 extend into the cavity to support a center sphere 31; when the support pins 40 are in the retracted state, the end faces thereof define a portion of the inner wall of the cavity 3. Although not shown in the diagram, a total of six support pins 40 are provided at evenly spaced positions—three in the upper mold half 20a and three in the lower mold half 20b. This diagram schematically shows the structure of a mold for injection-molding a golf ball, although the scale, structure of details and the like differ from those of an actual mold. The same applies as well to the other accompanying diagrams described below.
In the above mold 10, runners 60 and resin gates 70 with channels of given sizes (areas) are formed along the parting surface of the mold body 20 so as to enable a known cover-forming material to be injected between the inner wall of the cavity 3 and the center sphere 31 from a known injection molding machine (not shown). Next, with injection of the cover-forming material, the support pins 40 that were extended into the cavity 3 are retracted, after which cooling is carried out, thereby completing molding of the cover.
However, when a cover is formed using the above mold 10, particularly in cases where a thin cover is formed, a molding problem of the sort described subsequently sometimes arises. This molding problem is described in detail below while referring to the accompanying diagrams.
FIG. 5 shows an enlarged cross-sectional view of the vicinity of the cavity 3 in the conventional mold 10 shown in FIG. 4. For the sake of simplicity, the cross-section shown in FIG. 5 does not include certain elements which appear in FIG. 4, such as the support pins 40 and resin gates 70. FIG. 5A shows the state prior to the injection of resin into the cavity 3, and FIG. 5B shows the state while resin is being injected into the cavity 3, for a case in which a thin cover is to be formed. Also, in FIG. 5, the symbol Q represents poles of the inner wall of the cavity 3 in a vertical direction, and the symbol P represents poles of the inner wall of the cavity 3 in a horizontal direction. The latter poles P lie on the equator of the inner wall of the cavity 3.
In FIG. 5A, the center sphere 31 is held at the center of the cavity 3 by support pins (not shown). At this time, the center sphere 31 has a spherical shape, the inner wall of the cavity 3 is a spherical surface, and the interval between the two has been set so as to be everywhere uniform. Consequently, a vertical diameter connecting the two poles Q, Q in the vertical direction of the inner wall of the cavity 3 and a horizontal diameter connecting the two poles P, P in the horizontal direction are each the same length. The vertical diameter and horizontal diameter are both lengths measured at a surface defined by the inner wall of the cavity 3 were it assumed to have no dimple-forming protrusions thereon (i.e., when the shape of the cavity is a spherical surface, the hypothetical spherical surface defined by the inner wall of the cavity were it assumed to have no dimple-forming protrusions thereon).
Next, when resin is injected into the cavity 3, the resin flows in from the resin gates (not shown) formed along the parting surface and, as shown in FIG. 5B, fills the gap between the inner wall of the cavity 3 and the center sphere 31. At this time, because pressure from the horizontal direction acts upon the center sphere 31, the sphere 31 changes in shape from a true sphere to an approximate ellipsoid which is longer in the vertical direction. Hence, the interval between the inner wall of the cavity 3 and the center sphere 31 widens near both poles P, P in the horizontal direction, whereas the interval between the inner wall of the cavity 3 and the center sphere 31 narrows near both poles Q, Q in the vertical direction. As a result, the cover that is formed over the center sphere 31 becomes thicker near the poles P, P in the horizontal direction and becomes thinner near the poles Q, Q in the vertical direction, and thus has a thickness which differs depending on the position. Moreover, when the interval between the inner wall of the cavity 3 and the center sphere 31 becomes too narrow near the poles Q, Q in the vertical direction, resin may not sufficiently fill the gap near the poles Q, Q in the vertical direction, which may result in molding defects.
Separately, in connection with the ball structure, a desire exists for the formation of thinner covers in order to lower the amount of backspin on shots with driver. In cases where a conventional mold 10 is used to mold a thin cover, the interval between the inner wall of the cavity 3 and the center sphere 31 must be narrowed to accord with the thickness of the cover. However, for the reasons described above, either it has been impossible to achieve the desired quality or molding defects have arisen, making production difficult.
Up until now, this problem has been addressed by changing the molding conditions, such as increasing the injection speed and pressure and increasing the molding temperature (increasing the resin flow properties). However, excessively increasing the injection speed and pressure sometimes leads to a new problem in that the amount of molding flash increases, making the use of grinding as a finishing operation more difficult. If, instead, the molding temperature is increased, discoloration of the resin may occur. Particularly in the case of resins which have been imparted with a non-white color, the color sometimes fades, becoming lighter. As a result, the above problem has yet to be fundamentally resolved.
In the prior art, JP-A 2006-212057 discloses a way of forming a thin cover that entails lowering the injection pressure which acts upon the core during molding by providing a molten resin retractor at the gates of the mold. Methods for suppressing eccentricity of the center sphere include JP-A 10-328329, which discloses a production method that uses a compression mold having a rugby ball-shaped center cavity; and JP-A 10-508807, which discloses a production method wherein an elongated preform is created by injection molding, following which the preform is compression molded and ultimately finished to a spherical shape. However, in the above prior art, either mold fabrication is complex or many steps are required to obtain the finished product. Hence, there remains room for further improvement.
As shown above, various modifications have hitherto been made in order to improve the moldability and quality of golf balls, but a fundamental solution has yet to be found for the problem described above. Accordingly, to further enhance golf ball moldability and quality, a need has existed for a novel approach which is capable of resolving this problem.